pci: fix pci bridge fw path

pci: fix pci bridge fw path

qemu uses "pci" as name for pci bridges in the firmware device path.
seabios expects "pci-bridge".  Result is that bootorder is broken for
devices behind pci bridges.

Some googling suggests that "pci-bridge" is the correct one.  At least
PPC-based Apple machines are using this.  See question "How do I boot
from a device attached to a PCI card" here:
	http://www.netbsd.org/ports/macppc/faq.html

So lets change qemu to use "pci-bridge" too.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>